Re: [PATCH 1/2] arm64: Rename WORKAROUND_1165522 to SPECULATIVE_AT

From: Mark Rutland <mark.rutland@arm.com>
To: Steven Price <steven.price@arm.com>
Cc: Marc Zyngier <maz@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] arm64: Rename WORKAROUND_1165522 to SPECULATIVE_AT
Date: Mon, 11 Nov 2019 17:16:35 +0000	[thread overview]
Message-ID: <20191111171621.GA30274@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <013eec05-b558-d97a-bf95-248a62f25dc5@arm.com>

On Mon, Nov 11, 2019 at 04:07:39PM +0000, Steven Price wrote:
> On 11/11/2019 15:42, Marc Zyngier wrote:
> >> +config ARM64_WORKAROUND_SPECULATIVE_AT
> >> +    bool
> >> +
> >>  config ARM64_ERRATUM_1165522
> >>      bool "Cortex-A76: Speculative AT instruction using out-of-context
> >> translation regime could cause subsequent request to generate an
> >> incorrect translation"
> >>      default y
> >> +    select ARM64_WORKAROUND_SPECULATIVE_AT
> > 
> > I'd object that ARM64_ERRATUM_1319367 (and its big brother 1319537)
> > are also related to speculative AT execution, and yet are not covered
> > by this configuration symbol.
> 
> Good point.
> 
> > I can see three solutions to this:
> > 
> > - Either you call it SPECULATIVE_AT_VHE and introduce SPECULATIVE_AT_NVHE
> >   for symmetry
> 
> Tempting...

FWIW, this sounds fine to me.

> > - Or you make SPECULATIVE_AT cover all the speculative AT errata, which
> >   may or may not work...
> 
> This actually sounds the neatest, but I'm not sure whether there's going
> to be any conflicts between VHE/NVHE. I'll prototype this and see how
> ugly it is.
> 
> > - Or even better, you just ammend the documentation to say that 1165522
> >   also covers the newly found A55 one (just like we have for A57/A72)
> 
> Well Mark Rutland disliked my initial thoughts about just including both
> errata in one option like that - hence the refactoring in this patch.
> Although of course that's exactly what's happened with 1319367/1319537...

My view on this is that using one erratum config symbol to cover the
workaround for another is more confusing than having a level of
indirection, and I would've preferred the indirection for that case too.

> > What do you think?
> 
> I'll have a go at SPECULATIVE_AT covering both VHE/NVHE - from an
> initial look it seems like it should work and it would be neat if it
> does. In particular it should avoid the necessity to require VHE when
> the erratum is present.
> 
> Otherwise I guess SPECULATIVE_AT_{,N}VHE is probably second best.

Both sound good to me.

Thanks for dealing with this!

Mark.